1. Field of the Invention
This invention relates generally to electrical switching apparatus and, more particularly, to circuit interrupters, such as, for example, aircraft or aerospace circuit breakers. The invention also relates to methods of interrupting overcurrents of a power circuit.
2. Background Information
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
It is known to provide a cantilevered ambient compensation bimetal operatively associated with the bimetal. The bimetal, when heated, moves an insulative shuttle, which pulls on the ambient compensation bimetal that, in turn, is attached to a trip latch member. An increase or decrease in ambient temperature conditions causes the free end of the bimetal and the free end of the ambient compensation bimetal to move in the same direction and, thereby, maintain the appropriate gap between the two bimetal free ends, in order to eliminate the effects of changes in ambient temperature. Under overcurrent conditions, the bimetal and insulative shuttle pull on the ambient bimetal, which moves the trip latch member to trip open the operating mechanism.
Subminiature circuit breakers are used, for example, in aircraft or aerospace electrical systems where they not only provide overcurrent protection but also serve as switches for turning equipment on and off. Such circuit breakers must be small to accommodate the high-density layout of circuit breaker panels, which make circuit breakers for numerous circuits accessible to a user. Aircraft electrical systems, for example, usually consist of hundreds of circuit breakers, each of which is used for a circuit protection function as well as a circuit disconnection function through a push-pull handle.
Typically, subminiature circuit breakers have provided protection against persistent overcurrents implemented by a latch triggered by the bimetal responsive to I2R heating resulting from the overcurrent. There is a growing interest in providing additional protection, and most importantly arc fault protection.
During sporadic arc fault conditions, the overload capability of the circuit breaker will not function since the root-mean-squared (RMS) value of the fault current is too small to actuate the automatic trip circuit. The addition of electronic arc fault sensing to a circuit breaker can add one of the elements required for sputtering arc fault protection—ideally, the output of an electronic arc fault sensing circuit directly trips and, thus, opens the circuit breaker. See, for example, U.S. Pat. Nos. 6,710,688; 6,542,056; 6,522,509; 6,522,228; 5,691,869; and 5,224,006.
U.S. Pat. Nos. 6,864,765, 6,813,131, 6,710,688, 6,650,515, and 6,542,056 disclose a circuit breaker including three different trip modes, all of which employ a trip latch to actuate an operating mechanism and trip open separable contacts. The three trip modes include: (1) overcurrent conditions (thermal trip) detected by a bimetal, which actuates a trip latch through a shuttle and an ambient compensation bimetal; (2) arc fault (and/or ground fault) conditions detected by electronic circuits, which energize a trip motor to actuate the trip latch; and (3) relatively high current conditions (instantaneous trip) also attract the trip latch.
U.S. Pat. No. 7,170,376 discloses a miniature coil assembly including a coil controlled by an arc fault detection circuit and a plunger. An elongated ambient temperature compensating bimetal is interlocked to an ambient temperature slide, whereby lateral movement of such slide is controlled, in part, by the ambient temperature compensating bimetal. The plunger is coupled to the ambient temperature slide, in order to effect an arc fault trip function therewith.
If a circuit breaker operating mechanism does not open the separable contacts relatively quickly, then the internal components of the circuit breaker may be damaged. For example, it is known that separable contacts can weld closed if an overcurrent or fault condition persists for too long a time. Furthermore, an excessive trip time can produce carbon when the separable contacts break the power circuit. This carbon may cause dielectric breakdown after the fault and allow a current carrying path when the circuit breaker is intended to be open. Also, installed circuit breakers may become corroded, stuck or otherwise damaged. This can cause major changes in the ability of the circuit breaker to protect the corresponding power circuit against thermal overloads.
A known circuit breaker includes a fusible link to prevent the fusing of the separable contacts and, thus, the inability to break the power circuit. The fusible link opens if the separable contacts weld or if a dielectric breakdown occurs.
There is room for improvement in electrical switching apparatus such as circuit interrupters.
There is also room for improvement in methods of interrupting overcurrents of a power circuit.